An example of the prior art superconductive wiring is shown in FIG. 1. The structure of the wiring is disclosed in the paper entitled as "Technology of Superconductive Wiring on Alumina Substrate", Electronic Parts and Material published by Association of Industrial Investigation, 1987 August, pages 89 to 92.
Referring to FIG. 1, reference numeral 1 designates a high purity alumina substrate, and the high purity alumina substrate is sometimes abbreviated as FGA (Fine Grained Alumina) substrate. On the high purity alumina substrate 1 a thick film wiring 2 is formed. The process of forming the thick film wiring 2 starts with preparation of a powder of yttria represented by the molecular formula of Y.sub.2 O.sub.3, a powder of the barium oxide represented by the molecular formula of BaO and a powder of the copper oxide represented by the molecular formula of CuO. These powders are calcined to produce a bulk solid of a superconductive ceramic material. The bulk solid of the superconductive ceramic material is pulverized to produce a powder of the superconductive ceramic material, and the powder of the superconductive ceramic material is mixed into an organic vehicle, so that a paste is formed. The paste is printed on the high purity alumina substrate 1 by using a screen printing technique, and, thereafter, the high purity alumina substrate is placed in the atmospheric ambient at about 800 degrees to about 1000 degrees in centigrade for sintering, then a thick film wiring strip 2 being formed.
However, a problem is encountered in the quality of the substrate usable. Namely, even if the paste of the superconductive ceramic material is printed on a low purity alumina substrate available in the market and a thick film wiring strip is, then, produced through a process sequence similar to that described above, the thick film wiring strip does not show any superconductivity. This is because of the fact that the low purity alumina substrate available in the market contains a large amount of impurities such as silicon dioxide. It is figured that the impurities react on the paste during the sintering stage and, for this reason, an occurrence of the superconductivity is suppressed in the thick film wiring strip. Such a phenomenon is reported in JAPANESE JOURNAL OF APPLIED PHYSICS, 1987 May, VOL. 26 No. 5, Table 1 in page L761.
It is therefore an object of the present invention to provide a structure of a superconductive wiring feasible for fabrication on a low purity substrate available in the market.